Conventionally, an uninterruptible power supply device is widely used as a power supply device for stably supplying ac power to a fundamental load such as a computer system. As shown in, for example, Japanese Patent Laying-Open No. 2001-61238 (Patent Literature 1), an uninterruptible power supply device generally includes a converter converting ac power into dc power, an inverter converting dc power into ac power, and a power storage device storing dc power. In a normal state, the converter converts ac power from a commercial ac power supply into dc power, and supplies dc power to the inverter while charging the power storage device. The inverter converts dc power into ac power for supply to a load. In the case of a power interruption of the commercial ac power supply, electric power is supplied from the power storage device to the inverter, so that the inverter continues supplying ac power to the load.